Power supply circuit for a heating element

ABSTRACT

An improved high-efficiency power supply for a heating element includes charge, discharge, and comparator circuits. The comparator circuit provides the proper duty cycle and controls the power to the heater for keeping its temperature nearly constant over a wide range of input voltages. Changing the value of the components of the charge and discharge circuits alters the power supplied to the heating element.

United States Patent 11 1 [111 3,794,808 Takagi 51 Feb. 26, 1974 [54]POWER SUPPLY CIRCUIT FOR A HEATING 3,432,737 3/1969 Hunter et al 321/2ELEMENT FOREIGN PATENTS OR APPLICATIONS [75} Inventor: susumu Takag"Tokyo Japan 262,286 1/1970 U.S.S.R 219/497 [73] Assignee:Yokogawa-Hewlett-Packard Ltd., 801,194 12/1968 Canada 219/497 Tokyo,Japan [22] Filed: Aug. 2, 1972 Primary ExaminerBernard A. GilheanyAssistant ExaminerF. E. Bell [21] Appl' Attorney, Agent, or Firm-A. C.Smith [30] Foreign Application Priority Data Aug, 5, 1971 Japan 46-59261[57] ABSTRACT [52] U S 219/497 219/494 An improved high-efficiency powersupply for a heat- [511 HOSb 1/02 ing element includes charge,discharge, and compara- [58] Fieid 307/33 tor circuits. The comparatorcircuit provides the 307/34 516 1 19 T 3'21 proper duty cycle andcontrols the power to the heater for keeping its temperature nearlyconstant pver a wide range of input voltages. Changing the value of [56]References Cited the components of the charge and discharge circuitsUNITED STATES PATENTS alters the power supplied to the heating element.

3,588,666 6/1971 Bertolini et a1 307/33 3 Claims, 3 Drawing FiguresPAIENTEU 3.794.808

sum 1 or 2 FIG I E l N 1 9 l u.

PATENIED FEBZ 5 3, 7 94. 8 O8 8MB 2 UF 2 FIG. 3

POWER SUPPLY CIRCUIT FOR A HEATING ELEMENT BACKGROUND AND SUMMARY OF THEINVENTION Previously, power supply circuits for heating elementsmaintained constant power to the heating element by using a seriesregulator. These circuits had low efficiencies because the suppliedpower used in the heating element was dissipated in the seriesregulator.

The limitations on the series regulator that maximum desired heatingpower must be supplied under minimum supply voltage, and that the powerfor a heating element be several watts caused difficulties. In batterypowered devices, the power dissipated in the series regulator shortensthe operating time.

Accordingly, one aspect of the present invention is that it utilizes thefull supply voltage. Power to the heating element is controlled byautomatically changing the duty cycle to corres-pond with changes in thesupply voltage.

A resistor and a capacitor, either or both of which may be variable,allow selection of the desired heating power. The resistor and capacitorare connected in series with the power supply. A switch is inserted inparallel with the capacitor to discharge the charge on the capacitorperiodically.

A comparator circuit compares the voltage applied to the capacitor witha reference voltage and provides an ON-OFF signal to the heater.

DESCRIPTION OF THE DRAWINGS FIG. I is a schematic diagram of thepreferred embodiment of the power supply circuit for a heating element.

FIG. 2 is two graphs. FIG. 2(A) shows the operating voltage waveformsapplied to the capacitor, while FIG. 2(B) shows the corresponding powersupplied to the heater in the circuit of FIG. 1.

FIG. 3 is a graph showing the variation of power regulation ratio in thecircuit of FIG. 1 with changes in supply voltage.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the schematicdiagram of FIG. 1, there is shown a preferred embodiment of the powersupply circuit for a heating element. A variable resistor and acapacitor are connected in series to a power supply 11 for the heatingelement 29. Switch 17 which may be of various constructions includingmechanical and electronic is connected in parallel with thecapacitor'and is opened and closed periodically with constant timeinterval (T) by a switch driver 19. Switch driver 19 may be amultivibrator or other well known circuit. Closing the switch 17discharges the capacitor 15. A comparator circuit 2] compares thevoltage Vc applied to the capacitor with a reference'voltage KE. Voltagecomparator circuits are also well known and commonly used. KE issupplied by a constant voltage supply 23 of voltage E and apotentiometer 25. A switch 27 is connected in series with the heatingelement 29 and the power supply 11. Switch 27 is driven by the output ofthe comparator circuit. The state of switch 27 is as follows:

Vc s KE Switch 27 ON Vc KE Switch 27 OFF In operation, the voltage ofthe capacitor Vc in the circuit of FIG. 1 goes up to the supply voltageV with time constant RC where R is the resistance of resistor 13, and Cis the capacitance of capacitor 15. After the lapse of the constant timeinterval T following closure of switch 17 at time t Vc is driven to zerovoltage level. The effect of the closure of switch 17 is illustrated inFIG. 2. The comparator circuit compares the voltage V0 with thereference voltage KB and closes the switch 27 until Vc reaches KE attime t,.

Accordingly, T, (which is t,t is shown in equation (1) below:

T is proportional to the ON time of the switch in periodical constanttime interval T. Therefore, the average heating power P is shown inequation (2) subject to the condition that the heating element hasconstant resistance.

Using equation (1) and (2), equation (3) is derived.

P CRV log (I KE/V) Defining a parameter beta ([3) as:

V BE

the heating power regulation ratio versus change in supply voltage inthe circuit of FIG. 1 can be expressed as:

The heating power regulation ratio P is shown in FIG. 3 as a function ofboth B and K.

FIGS. 2(A) and 2(B) also illustrate the effect of changes in the powersupply voltage V. If V is the new voltage frompower supply 11, then t,is the time when the voltage applied to the capacitor is equal to therefcrence voltage KE while T t, t

From FIG. 3, it can be seen that very nearly constant power regulationfor the heating element is obtainable vwith the proper values of K andE, i.e., proper reference voltage. From equation (3), it is evident thatthis invention permits variations in the heating power setting bychanging the values of capacitance and resistance in the charge anddischarge circuits.

I claim:

1. A power supply for a heating element comprising:

a heating element;

a power source for energizing the heating element;

a first switch means for repetitively connecting and disconnecting witha controllable duty cycle the heating element and the power source;

means for generating a substantially constant reference voltage;

sampling means for sampling the voltage of the power source; and

a comparator means for comparing the sampled voltage from the powersource with the constant reference voltage and increasing the duty cycleof the first switch means in response to decreases in sampled voltagefrom the power source relative to the constant reference voltage and fordecreasing the duty cycle of the first switch means in response toincreases in sampled voltage from the power source relative to theconstant reference voltage.

2. A power supply as in claim 1 wherein said comparator means includes:

a resistor and a capacitor serially connected with said power source;

second switch means connected across the capacitor;

age divider connected across a constant voltage supply; k i

UNITED STATES PATENT OFFICE I CERTIFICATE OF CORRECTION latent No. ,7 ,8Da February 26, 1974 Inventor'(s) Susumu Takagi It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Co1umn 2 line 24, Equation (2) should read:

"1 e v (T /T) v T Column 2-,Qline 29,; Equation (3) should read:

7 P -C R V log (1 KE/V) I I Signed and sealed this 18th day of June1971]..

,'(SEAL) Atteat: I I I EDWARD u.FLETcHER',.m. 1 c. 'mnsrmu. mum

Attoatzlng Officer. I I comisaio ar of Patents UNITED STATES PATENTOFFICE CERTIFICATE OF CORRECTION Patent No. 3,794 Dated Februs ry .26,1974 Inventor(s) Susumu Takagi It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 2 line 24, Equation (2) should read:

P a v (T /T) ==v 'r Column 2 line 29, Equation (3) should read:

P -CRV log (1 KE/V) Signed sealed this 18th day of June 1971;.

(SEAL) Atteat: n

EDWARD M.F'LE1GHER,JR. c. mnsxmm. mum

Atteating Officer. I r Comisaioner of Patents

1. A power supply for a heating element comprising: a heating element; apower source for energizing the heating element; a first switch meansfor repetitively connecting and disconnecting with a controllable dutycycle the heating element and the power source; means for generating asubstantially constant reference voltage; sampling means for samplingthe voltage of the power source; and a comparator means for comparingthe sampled voltage from the power source with the constant referencevoltage and increasing the duty cycle of the first switch means inresponse to decreases in sampled voltage from the power source relativeto the constant reference voltage and for decreasing the duty cycle ofthe first switch means in response to increases in sampled voltage fromthe power source relative to the constant reference voltage.
 2. A powersupply as in claim 1 wherein said comparator means includes: a resistorand a capacitor serially connected with said power source; second switchmeans connected across the capacitor; means for recurringly actuatingthe second switch means to discharge the capacitor; and comparator meansfor comparing the voltage across the capacitor with that of the constantreference voltage, for closing said first switch means in response tothe voltage across the capacitor being not greater than the constantreference voltage, and for opening said first switch means in responseto the voltage across the capacitor being greater than the constantreference voltage.
 3. A power supply as in claim 1 wherein said constantelectrical power source means includes a variable voltage dividerconnEcted across a constant voltage supply.